Method of separating lead and cadmium from zinc ores



Patented Maya 193 OFFICE V METHOD or SEPARATING LEAD AND GAD- MIUM FROM zmo ones Reed w. Hyde, Summit, IN. 1., assignor to Dwight & Lloyd Sintering Com v I -n 1 v N. Y., a corporation of Delaware No Drawing. Application April 21, 1937,

' Serial No. 138.105

3 Claims- (Cl. 15-1 My invention relatesto a process for separating lead and cadmium from zinc-containing ores or concentrates by roasting the ore or concentrate .on a sintering machine while passing a blast 5 of air downwardly therethrough.

i In the roasting of ores or ore concentrates con taining zinc. lead and cadmium compounds such as sulphides of these metals. it is customary to spread the ore or concentrate on the'traveling grate 01 a sintering machine. then ignite the ore and burn or roast it by a down-draft so that the lead and cadmium are volatilized while the zinc content of the ore remains as a residue. A suitable chloride, such as sodium chloride, may be added to the ore or ore concentrate to aid in the volatilization of the leadand cadmium. The ore or concentrate is sumciently' porous to permit the passage of air, having pores of amagnitude of about one-thirty second of an inch or less through which the air of the blast passes. One difilculty encountered in the operation of this process is that a very considerable quantity of lead and cadmium remains in the residue, although the Y temperatures-employed may be sumcient to coma pletely volatilize the lead and cadmium compounds.

This difllculty is avoided in my invention by which the lead and cadmium may be completely or. almost completely removed, leaving only a very minute quantity of these metals in the residue remaining after the roasting. y

In my invention I support the bed of ore or ,concentrate being roasted in such a manner that the removal of heat by conduction from the bot-.

tom surface of the bed is reduced to a very small or negligible quantity and at the same time provide relatively large or wide passageways for the gases carrying the lead and cadmium compound or fume. By treating the bed of ore in thismanner the maximum roasting temperature may be carried to the lower surface of the bed thereby keeping the lead and cadmium in the vapor state or phase rather than as a suspended fume until it is entirely free from the roasting bed. Rela- 5 tively coarse passages are provided-tor the gases after they have left the roasting bed so that as they cool the lead and cadmium which condenses as-fume is not brought into intimate contact with surfaces upon which it may condense and collect. 50 For this purpose I provide between the metallic- .supporting pallets oi the sintering machine and the material being roasted a layer of low heat conductivity and or large passageways for the gases of combustion. This layer may be a top 55 covering attached to the pallets, as for example,

a heat resistant ceramic material having suflicientiy large pores. or it may be a coarsely crushed layer of heat-resistant material of low conductivity such as the sintered residue formed in a previous roasting operation and which had been crushed to such a size as to give the proper air passages.

' 'Iheuse of this material has the advantage that it may be collected along with the sintered prod-.

not without the necessity for separating the two. 10 This product dilfers irom the lower layer of the bed of ore or concentrate being roasted in that it has much larger air passages.

Consequently, although its temperature may be much lower than that of the roasting bed and 15 therefore such that volatilized lead and cadmium will condense as a suspended fume, the suspended particles of fume thus'formed pass freely through the intervening supporting bed and leave it substantially free i'rom cadmium and lead. .As a

result, the roasting temperature may extend throughout every part of the bed having the small pores of the concentrate being roasted, to free these pores of the lead and zinc content. The average amount 01 lead and cadmium remaining in the roasted ore is thereby reduced to a negligible quantity. Another diilerence is that whereas a slight deposition of lead and cadmium fume in the narrow passages of the roasting bed causes a proportionately very great narrowing 01 0 these passageways and thus aggravates the tendency to deposit the fume, any deposition of such fume on the relatively coarse passages of the insulating supporting layer has a negligible eflect so far as narrowing the passage areas is 35 concerned.

As an example of the process, a layer of coarsely crushed sintered material of one-quarter inch to one-half inch or one-quarter inch to threequarter inch size is provided as a covering layer 40 for the pallets of the sintering machine. v, The material crushed to this size provides voids or passages approximating about one-eighth inch to one-sixth inch in diameter. On this grate cover-- ing there is placed a layer of ore or ore concentrate having passages of a maximum dimension up to about one-thirty-second of an inch. The ore is then ignited at the top surface, some fuel being supplied it necessary, and burned with a down-draft so that a zone of combustion of maximum temperatures passes from the top to the bottom layer of the ore bed. This combustion zone is sufliciently high in temperature to volatilize the lead and cadmium. contimgmsly driving it into the lower cooler areaswhere any dewardly until it reaches the lower surface of the ore bed. 'I'hereupon it passes into the relatively coarse passages through which it is rapidly car-= ried without undue loss of temperature or deposi tion of fume.

By the above process a concentration oi. lead and cadmium in the lower part of the ore bed being roasted is avoided and a residue of substantially uniform composition is obtained. For example, a sintered bed roasted in the manner heretofore used had an average lead content of 0.15% and an average cadmium content of 0.015%. The concentrations of lead and cad mium, respectively, in the lower layers of the bed were as high as 0.25 and 0.175 indicating a very rapid increase in lead and cadmium content toward the lower surface. In contrast, a sintered bed formed by my process hadan average lead. content of 0.02% and an average cadmium content of 0.002% and the concentration of lead and cadmium at the bottomJayer were respectively 0.025% and 0.003%. This percentage is not substantially increased .when this sintered material is coarsely crushed and used as a supporting layer for a subsequent bed to be roasted. Consequently, the fresh sinter and that used as insulating sup porting; layer may be commingled in the final product.

30 Through my invention, therefore, I have provided a process whereby the lead and cadmium may be separated substantially completely from zinc by a relatively simple roasting operation.

9,1 races draft sintering machine, the method oipreventing concentration of the lead and cadmium in the lower portion of the sintering charge which comprises providing air passages in the bottom of said charge ranging from ,4; to V inch, the diameter of the air passages through the upper portion of the charge having a maximumoi about 1% of an inch.

3. In the blast aalntering oi zinciferouamate rials containing lead andcadmium. in a downdraft sintering machine, a protective hearth layer consisting of material of relatively low heat conductivity and. of a particle size adapted to leave air passageways through the layer having diameters approximately V to V inch, and a sintering charge of said zinciferous materials overlaying said hearth layer having air passageways not ex ceeding about 1 inch in diameter.

Reno W. ma. 

